This invention relates to apparatus for measuring a physiological pressure, particularly a blood pressure, and more specifically, the invention relates to an electrical calibration system for such blood pressure monitoring apparatus.
In blood pressure monitoring apparatus to which the invention is directed, a catheter is inserted into a patient's blood vessel, the catheter being connected by tubing to a pressure transducer. A saline solution normally fills the tubing between a catheter and the transducer. The liquid in the system applies a pressure, directly related to the patient's blood pressure, to a diaphragm in the transducer. A Wheatstone bridge of four strain gauge resistors is formed on a silicon chip that is connected mechanically or through a fluid to the transducer diaphragm to produce a change in resistance dependent upon the flexing of the diaphragm.
The transducer is connected to a monitor which provides a continuous real time display of variations in the patient's blood pressure as provided by the transducer bridge. Part or all of the transducer is disposable. One form is a completely disposable transducer unit which receives the saline solution in one part and provides the electrical signal from another part. Another form provides a reuseable transducer section and a disposable transducer dome which is mounted on the transducer section. The saline solution which can become contaminated is isolated from the transducer in the reuseable section by a diaphragm mounted on the disposable transducer dome.
Substantially all systems provide for a calibration check. In that check, a resistance is connected, by a switch, across one leg of the bridge transducer in such a fashion that the monitor will show a pressure of 100 mmHg if the electrical connections are proper. In most hospitals where apparatus of this type is used, it is part of the daily or new shift procedure to close that switch to determine whether the apparatus is functioning properly. If there is no reading whatsoever, an open circuit has been introduced into the system. If the reading varies significantly from 100 mmHg, something has happened to cause extraneous resistance to creep into the system.
Many monitors have a switch on the monitor itself by which the shunting resistance is connected across the bridge. U.S. Pat. No. 4,603,574 discloses resistance that is mounted on the cable, as part of the transducer assembly, with series switches provided to make the shunt connection.